Go outside tonight and look between the stars. It is dark. That plain fact is a real puzzle: in an endless, unchanging universe, there would be no dark gaps at all.
Stand in a thick forest and look in any direction. You never see far, because every line of sight ends on a tree trunk. Now swap the trees for stars. If the universe went on forever and was full of stars everywhere, every line of sight would end on a star too. The whole sky would shine like the surface of a star, about as bright as the Sun's disc, from horizon to horizon.
Distance does not rescue you. A star twice as far looks four times dimmer. But picture a thin shell of space around you. The shell twice as far out has four times the volume, so it holds four times as many stars. Four times the stars at a quarter of the brightness each: the same light. Every shell adds the same glow, and endless space has endless shells.
Here is a flat toy universe. You are at the centre. Stars are scattered at random. Around the edge is your sky: a ring of tiny slices, one per direction. A slice lights up when your line of sight hits a star that light has had time to reach. Drag the gold handle outward, or press play, to let light travel further.
Gold wedges are directions that end on a star. Dark wedges are directions where light from beyond the horizon has not arrived yet. Dim dots are stars whose light is still on its way.
Try it with few stars first. With a modest crowd the sky fills in surprisingly gently, then the last dark gaps vanish. The share of sky that is still dark falls off like a decaying exponential in how far you look. Give the light enough time and even a thin scatter of stars paints the whole ring gold.
Johannes Kepler used the dark night sky in 1610 as an argument that the universe must be finite. Edmond Halley and Jean-Philippe Loys de Chéseaux turned it into a sharper argument in the 1700s. Heinrich Olbers described it in 1823, and his name stuck even though historians point out he was not the first. In 1848 Edgar Allan Poe guessed the answer in his prose poem Eureka. Lord Kelvin worked it out properly in 1901.
The answer is hiding in the slider you just moved. The universe has a finite age, about 13.8 billion years, and light travels at a finite speed. So you can only see the stars whose light has had time to reach you. Beyond that horizon, the stars may be there, but their light is still on its way. Real stars do not shine forever either, so the glow never gets a chance to build into a wall.
The glowing wall is not entirely missing, though. If you look with a microwave telescope, the sky really is lit in every direction. That glow, the cosmic microwave background, comes from the hot young universe, about 380,000 years after the start, when the whole sky was bright. Space has expanded since then, stretching its light by a factor of roughly 1,100 until it became microwaves, and it now corresponds to a chilly 2.7 kelvin. The wall was real. We just can't see it with our eyes.
Drag the horizon back to the middle and notice how many directions are still dark. That is our sky: most lines of sight end in a stretch of space where, so far, nothing has had time to arrive.